In collecting blood for HPLC and other procedures as described in U.S. Pat. No. 4,057,499 of Buono, it is standard to centrifuge the blood in a standard tube having a closed lower end and a cylindrical upper portion. The whole blood is separated into the heavier solids in the lower closed end of the collection tube and a lighter liquid phase above it, normally separated at a clearly visible interface as the solids are normally opaque while the serum is clear. A separator or gel layer may lie at this interface.
A piston-like filter is used to further separate the blood solids, mainly cells, from the liquid phase, the serum or plasma. This filter fits in the collection tube with slight clearance and comprises a cylindrically tubular sleeve that fits in the tube and that carries on its lower end a porous membrane of an outside diameter that is slightly greater than the inside diameter of the collection tube. As the filter is pushed down in the collection tube the membrane forms a seal with the inner surface of the tube and allows air out as it is pushed down into contact with the surface of the serum floating atop the mass of solids. Once in contact with the liquid, the serum passes up through the filter, which might have a 0.2 .mu.m or 0.45 .mu.m pore size, leaving nothing but pure testable liquid in the chamber of the filter above the membrane.
Normally the sleeve of the filter is fairly short so that it can be advanced completely below the rim of the collection tube, and a cylindrical pusher that snugly fits in the collection tube is used to push it down. This is typically done manually with an operator watching that the membrane is stopped before it reaches the interface.
So long as the operator works carefully this procedure is relatively efficient. Problems occur when, for example, the filter membrane is pushed past the solid/liquid interface, contaminating the bottom of the filter with these particles. In addition when the sample has a high proportion of serum, the filter is sometimes pushed down so much that the liquid phase overflows the top of the filter sleeve, contaminating the pusher and generally making a mess so that when the filter is raised out of the centrifuge tube it sheds drips that can contaminate the workplace or other samples. It is very difficult for the operator to determine the upper surface level of the liquid when the filter is in place so overflowing it is very had to avoid with blood having a low solids content.